I liked some aspects of both games. I loved the variety of the Demon's Souls weapon upgrade system, though the more straightforward system in Dark Souls removed some of the unnecessary convolution.
The Dark Souls spell system was welcome, but I loved how you could use demons' souls to create new spells and miracles in Demon's Souls.
In general I like the refinements in the Dark Souls combat system, but I like the atmosphere of Demon's Souls a little more. However, the structure of Dark Souls's world is more conducive to exploration.